Back-face culling isn't an option in TG2 because back-face culling is always done where possible. It is equivalent to always being enabled. The only time it is disabled is if you enable "double-sided" on an imported object, and it is only disabled for that object.
Back-face culling only looks at the direction in which a surface is pointing, not whether it will be covered up by other parts of the scene. It is different from occlusion culling, whereby parts of the scene that will be covered up are not rendered. Terragen tries to do this by rendering front-to-back, but there are some situations where it finds it difficult to predict which parts are closer than others, because of the way it is often working with very large polygons and subdividing them as it renders. It may be possible to improve this in future by performing a pre-subdivision stage to the rendering. I intend to work on this in future.
Matt
